This invention relates generally to a vehicle braking system having an eddy current reduction retarder for imparting braking force to the vehicle, and more particularly, to a permanent magnet type eddy current reduction retarder apparatus of reduced weight and increased braking ability.
As disclosed in Japanese Patent Application Laid-Open No. 3(1991)-86.061, a permanent magnet support cylinder is received by an annular guide frame constituting a part of a protective case. During braking, the magnet support cylinder projects into a brake drum, and the permanent magnets form magnetic circuits between the magnets and the brake drum through ferromagnetic pole pieces of the guide frame. When the rotating brake drum traverses the magnetic fields, eddy currents are generated to produce a braking force in a direction reversed to the rotational direction of the brake drum.
However, in that conventional eddy current retarder device, the magnetic flux passes through thick ferromagnetic pole pieces resulting in a reduction in the magnetic flux arriving at the brake drum and a corresponding reduction in the braking force. A further reduction in braking force is caused by the large electric conductivity of the brake drum. In addition, since the ferromagnetic pole pieces are connected to the guide frame, the overall construction is complex, manufacturing cost is high and weight is increased.
An improved permanent magnet type eddy current reduction device is disclosed in U.S. Pat. No. 5,804,897. That device provides a non-magnetic (for example, austenite stainless steel) tubular element on a guide tube opposed to an inner peripheral surface of a brake drum to reduce weight of the guide tube while also preventing entry of dust or muddy water. However, in the aforementioned device, the air gap between the inner peripheral surface of the brake drum and the guide tube (strictly, the non-magnetic gap between the inner peripheral surface of the brake drum and the outer surface of the magnets) is so large that braking force is reduced.
The object of the present invention, therefore, is to provide an improved permanent magnet type eddy current braking apparatus which exhibits reductions in cost and weight and increased braking ability.